In known terrestrial cellular telecommunications systems the resources that enable a terminal (e.g., mobile phone, fixed terminal, etc.) to communicate on the system are fixed with respect to the terrain. Generally the terrain is divided into distinct cells 2 (FIG. 1) which may be grouped into location areas 9. By way of example, the borders of the location areas 9 are set forth in bold. The location areas 9 are grouped into mobile service switching centers (MSC) regions 6. The MSC regions 6 together constitute the service area of a Public Land Mobile Network (PLMN) 8.
Each cell is supported by a unique set of radio resources, including a radio tower. These resources are part of a Base Station Subsystem (BSS). The resources devoted to a single cell are supported by a Base Transceiver Station (BTS). The terrestrial area included in a cell is therefore dictated by the coverage capabilities of its associated radio tower. A location area is a set of cells which are treated as a common pool of radio resources for certain functions such as the paging of a terminal to notify it of an incoming call. That is, all cells in a location area would page the called terminal. By grouping cells into location areas, the system defines a larger terrestrial area than that supported by a single cell. Hence, a terminal is allowed to roam over a larger area and still have the cellular network consider its position as being `known`.
A location area, in turn, will belong to one and only one MSC region. A MSC region is the geographical area served by a Mobile-services Switching Center (MSC) (for example, MSCs 11 or 21 of FIG. 1) and its associated Visitor Location Register (VLR) (for example VLRs 13 or 19 of FIG. 1). MSCs and VLRs may be paired together. When it is not important to make a functional distinction between an MSC and a VLR, the pair is referred to as an MSC/VLR. The MSC is the point at which a cellular network interfaces its radio resources network into a traditional land-line based network. Also the MSC may interface with the Public Switched Telephone Network (PSTN), in which case it is referred to as a gateway-MSC (GW-MSC) 5.
When a terminal is purchased, it is assigned a mobile-services integrated services digital network (MSISDN) number, i.e., a mobile phone number, from the stock of numbers assigned to the cellular services provider. This number and subscriber service information is entered into a data base called a Home Location Register (HLR) 3. When a terminal is turned on, it searches the airwaves for a broadcast channel which is transmitting location area identification (LAI) information. Each BTS operates such a broadcast channel in its cell. The terminal receives the LAI information and compares it to the LAI stored in its memory. The LAI in the terminal's memory may be programmed by the terminal's vendor if it has just been purchased, or it may be the LAI of the location area of the terminal at the time that it was last turned off or left the services area. If the broadcast and memory-resident LAIs match, then the terminal goes into a standby mode and is ready to originate or terminate calls.
If the two LAIs do not match, then the terminal must re-register, for the network is not aware of the terminal's current location. The terminal registers by signaling (through the BSS) to the MSC/VLR whose region includes the terminal's current location area. For example, in FIG. 1, terminal 1 would register by signaling to MSC/VLR 11/13. The MSC/VLR 11/13 notes the terminal's current location area and determines whether the terminal is already registered with it in a previous location area in the same MSC region 6. If so, then the MSC/VLR 11/13 changes the terminal's location data, the registration ends, and the terminal enters a standby mode. However, if terminal 1 were to move from location area 9 to location area 4, the terminal 1 must re-register with MSC/VLR 21/19. Otherwise, neither MSC/VLR 11/13 nor MSC/VLR 21/19 will be able to access terminal 1 since MSC/VLR 21/19 will lack the necessary terminal identification and location information and since MSC/VLR 11/13 will be too remote to maintain an RF communications link with the terminal 1. To effect re-registration, the MSC/VLR 21/19 informs the terminal's HLR 3 that the MSC/VLR 21/19 is serving the terminal. The HLR 3 notes this information and checks to see whether the terminal 1 was previously registered with another MSC/VLR, such as MSC/VLR 11/13. If a previous registration existed, the HLR deletes this old registration and signals the MSC/VLR 11/13 holding the previous registration information to de-register the terminal.
The purpose of this exchange of information is to enable the routing of mobile-terminated calls (calls to the terminal) and to be able to identify the terminal when it places mobile-originated calls (calls from the terminal). If such re-registration did not occur, calls placed to terminal 1 would be lost since gateway 5 will route such calls to MSC/VLR 11/13 which represents the last known MSC/VLR registered with the HLR 3 for terminal 1. The use of the VLR and HLR information in PLMNs is described below.
Mobile-terminating calls enter the PLMN of the terminal subscriber at a GW-MSC. The method for routing the call to the GW-MSC may be any standard telephony practice based on the MSISDN number of the called terminal. The GW-MSC examines the called terminal's MSISDN number and determines which HLR serves the subscriber. Based on this information, the GW-MSC signals that HLR and requests information on how to route the call to the terminal. The HLR consults its data base and finds the MSC/VLR serving the terminal. This is why the MSC/VLR had to inform the HLR that the MSC/VLR was serving the terminal. The HLR informs the VLR that a call is pending for the terminal with the called MSISDN number, and the HLR requests a telephone number at the MSC to which the call can be routed. The VLR delivers that telephone number to the HLR, and the HLR passes it back to the GW-MSC. The GW-MSC routes the call to the MSC. When the call reaches the MSC, the MSC queries its associated VLR to determine the identity of the called terminal and the location area in which the terminal should be paged. This is why the terminal must inform the MSC/VLR whenever its location area changes. The VLR responds to the MSC with the terminal's identity and LAI. The MSC requests the BSS to page the terminal in the terminal's location area. The BSS sends this paging request on to the BTSs covering the cells in the terminal's location area, and these BTSs broadcast the page. The terminal in standby hears the page and responds. After a brief exchange of signals, the end-to-end call connection is complete.
Mobile-originating calls do not require so elaborate a routing mechanism. However, as a security measure against fraudulent use of the PLMN and to preserve the confidentiality of the mobile subscriber, the VLR can have information known only to it and the terminal. This information is constructed as part of the registration signaling between the terminal and the MSC/VLR and is stored at the terminal and in the VLR. When a mobile-originated call is initiated, this information must be present in the VLR so that fraud-prevention and confidentiality mechanisms can be implemented.
Note that terminal registration (other than the initial registration of a brand-new terminal) is caused by terminal movement from one location area to another. This movement is not coordinated among the terminals, and thus occurs randomly and at a relatively low rate.
Note also that the collecting of cells into location areas is an important trade-off in the detailed design of a cellular system. Large location areas reduce the number of terminal registrations because terminals have to travel farther before they leave their current location area. Since registration consumes radio signaling resources, lowering the number of registrations tends to increase the capacity of a PLMN with a given amount of radio resources. On the other hand, terminals must be paged across their entire location area since a terminal is free to move around inside its location area without informing the network of its movement (via registration). As a location area is made larger, the terminals within it must be paged in more cells. Since paging also consumes radio signaling resources, decreasing the size of location areas tends to increase the capacity of a PLMN with a given amount of radio resources. Thus, it is desirable to find an acceptable size for location areas so that a minimum of radio signaling resources are employed for the joint duties of registration and paging.
The foregoing operation is manageable in conventional terrestrial systems since re-registration is dictated by movement of individual terminals between location areas. Hence, terminals re-register individually. The terrestrial system never requires simultaneous mass re-registration of a large number of terminals.
However, satellite based systems experience difficulties not addressed in terrestrial systems. Proposed satellite-based telecommunications systems include terminals, satellites, earth stations, MSCs/VLRs, GW-MSCs, and a terrestrial network interconnecting the earth stations, MSCs/VLRs, and GW-MSCs. The satellites may perform some functions related to the functionality of the BTSs and the earth station may perform some functions related to the functionality of the BSS.
Satellite-based telecommunications systems have been proposed which employ satellites orbiting the earth at other than geo-stationary altitudes. The satellites in these systems move with respect to the earth's surface, and so their fields of coverage on the earth's surface continuously change. In the analogy with PLMNs, it is as if the radio towers (the BTSs) were in continuous motion. The cells in cellular systems are defined by the reach of the network radio resources, and thus the cells in the satellite-based systems are in continuous motion. Since the cells are in continuous motion, so too are the location areas. Because, as shown above, a terminal must re-register whenever its location area changes, either the terminals would be continually registering or location areas would have to be very large.
Further, registrations caused by cell motion can have catastrophic consequences on the operation of the satellite-based systems. The root of the potential catastrophe is that the motion of the satellites affects many terminals in the same way simultaneously, or nearly simultaneously. When a satellite no longer covers a terminal, it is also not covering any other nearby terminals. The ground speed of the field of coverage of satellites in proposed satellite-based telecommunications systems is several kilometers per second. Thus, a great number of terminals may become uncovered by a particular satellite in a matter of seconds. If all of these terminals were to register to the radio resources in another satellite, that other satellite would be inundated with registration signaling. Such a near-simultaneous registration of terminals in a given geographic area is termed a "mass registration event." At the very least, the satellite's traffic capacity would fall by the amount of signaling required for the registrations. More likely, all available signaling channels would be choked with registration signaling, and no new calls either to or from terminals in the area could be initiated during registration.
To further illustrate the concept of mass re-registration, reference is made to FIG. 2 which illustrates how changing satellite-to-terminal connectivity can trigger re-registrations between times t.sub.1 and t.sub.2. At time t.sub.1 satellite 23 covers terminals in area 25. At time t.sub.2 satellite 23 moves beyond area 25. According to the aforementioned process, when satellite 23 moves, terminals in area 25 registered through satellite 23 at earth station 27 must re-register with earth station 29 or be lost to the system.
Moreover, a mass registration event may occur even while a collection of terminals in a region remains under the coverage of a single satellite. In order for a satellite-based telecommunications system to function, the satellites must remain in contact not only with the terminals but also with the network. The contact points for the satellites are earth stations. From time to time as it orbits, each satellite breaks contact with one earth station and establishes contact with others. Conventional satellite-based telecommunications systems include MSC/VLRs as integral parts of each earth station. A satellite must be in contact with an earth station in order to be in contact with the MSC/VLRs in that earth station. Thus, when a leaving satellite breaks contact with the earth station it also breaks contact with the uncovered earth station's MSC/VLR. Hence, when the system routes terminal-terminated calls covered by the leaving satellite to an MSC/VLR no longer covered by the satellite, the uncovered MSC/VLR can no longer put calling parties in contact with the termination terminals. All of the terminals registered at the uncovered MSC/VLR and in standby under the satellite have effectively lost contact with the network. In order to regain contact, they must all register with some other MSC/VLR which is still in contact with the covering satellite(s). This mass re-registration event would again choke the satellites' signaling resources and either greatly diminish or altogether eliminate the capacity for new traffic.
FIG. 3 illustrates how satellite-to-earth station connectivity can trigger re-registrations. In FIG. 3, terminal 31 is registered in MSC/VLR 33 of earth station 35. As satellite 37, the only satellite covering terminal 31, moves overhead, it breaks contact with earth station 35. At that point, terminal 31, and all similarly registered neighbors, must re-register with earth station 39, or again, be lost to the system.
It is not practical to enlarge location areas in satellite-based telecommunications systems so that registration is not caused by satellite motions. Several proposed systems have low- and medium-earth orbit satellites whose fields of coverage move across the entire earth in less than a day. Enlarging location areas to a size to include all of the earth or very large portions of the earth would place an impractical paging burden on the radio resources of the satellites.
Thus, there is a need for a satellite based cellular telecommunications system that permits any location area size while limiting the number of re-registrations to only those necessitated by the movement of the terminals from one location area to another (no re-registrations caused by satellite motion). Further, the system should be inter-operable with current commercially-available MSCs, VLRs, MSC/VLRs, and GW-MSCs, so that its development cost permits commercially feasible implementation.